Polybutylene terephthalate and copolymers composed of at least 80% by weight of polybutylene terephthalate (hereinafter generically referred to as "PBT") have been used as engineering plastics for widespread applications, such as automobiles and electrical and electronic equipment, because they are crystalline thermoplastic resins exhibiting excellent mechanical and electrical properties as well as well as excellent physical and chemical characteristics.
PBT resin compositions compounded with organohalogen flame retardants or flame-retardant assistants for the purpose of imparting flame retardancy have been used previously for applications where flame retardancy is required, such as to from electrical components.
However, known PBT resin compositions compounded with flame retardants and flame-retardant assistants have the following problems.
(1) The presence of a flame retardant and an assistant lowers the physical properties of molded articles.
(2) PBT decomposes and deteriorates during the preparation of a composition or molding due to the interaction of a flame retardant and an assistant.
(3) A flame retardant and an assistant in addition to decomposition by-products thereof cause molds to corrode and stain so that the dimensional accuracy of molded articles and fabrication efficiency are lowered.
(4) A flame retardant and an assistant in addition to decomposition by-products thereof bring about the formation of spots on molded articles which spoil the appearance and performance of the same.
(5) Poor compatibility of a flame retardant with the resin brings about oozing of the flame retardant on the surface of a molded article to cause various problems.
(6) The flow of the resin is kindered, so that it becomes difficult to conduct molding.
(7) When a molded article is used, a flame retardant and an assistant used therewith or by-products of the decomposition thereof corrode or stain a metal in contact therewith or present in the vicinity thereof, which hinders the functions of electrical components. For example, when a molded article is used in electrical components having electric contacts made of a metal (e.g., housings or boards of switches, relays, etc.), there occurs various problems including a remarkable increase in the electrical resistance of the contact resulting from the staining of the contact metal during the long-term use thereof.
Various proposals have been made in order to solve these problems. However, it was very difficult to prepare a flame-retardant PBT composition capable of solving all of the above-described problems even though it was possible to solve part of the problems. Therefore, the development of a flame-retardant PBT composition capable of solving all of the above-described problems has eagerly been desired.
In general, the molding of polybutylene terephthalate is relatively easy as compared with that of polyethylene terephthalate. However, in recent years, there are many cases where a further reduction in the production cost of molded articles is desired. In particular, there is an increasing demand for the enhancement of productivity through shortening of the molding cycle (i.e., the so-called high-cycle molding). In general, the present invention is directed to a before "flame-retardant"; flame-retardant PBT composition which has excellent flame retardancy and well-balanced performance characteristics and is free from the above-described drawbacks. Particularly, the compositions of the present invention shorten the molding cycle and the realization of the high-cycle molding and causes no problems with respect to mold staining.
Specifically, the present invention relates to a flame-retardant polybutylene terephthalate resin composition comprising:
(A) polybutylene terephthalate or a copolymer composed of at least 80% by weight of polybutylene terephthalate;
(B) 0.5 to 25% by weight, based on the composition, of a halogenated bisimide represented by the general formula (1): ##STR2## wherein R.sub.1 is a divalent organic group and R.sub.2 and R.sub.3 are each a divalent organic group, provided that at least one of R.sub.2 and R.sub.3 has at least one halogen atom;
(C) 0.05 to 5% by weight, based on the composition, of a polytetrafluoroethylene resin; and
(D) 0 to 60% by weight, based on the composition, of an inorganic filler.
In addition, the present invention provides a PBT composition having desired performance and effects further improved by incorporating an olefinic polymer (E) and/or a stabilizer (F) to the abovementioned fundamental components.